Columnar grain ceramic thermal barrier coatings

ABSTRACT

A coated article and method for producing the coated article are described. The article is coated with a system which provides protection against oxidation and corrosion and which significantly reduces the substrate temperature. An MCrAlY layer is applied to the article to be protected and a columnar grain ceramic is applied by vapor deposition to the MCrAlY coated article. An alumina layer which exists between the MCrAlY layer and the columnar ceramic layer provides for the adherence of the columnar layer to the MCrAlY layer.

TECHNICAL FIELD

This invention is concerned with the field of ceramic coatings on metalsubstrates. The coating and method described in the present applicationare useful for the application of protective ceramic thermal barriercoatings to gas turbine engine components. Through the use of thepresent coating, substantial increases in turbine operating temperaturesmay be possible.

BACKGROUND ART

This application is similar in some respects to U.S. Ser. No. 109,955(Columnar Grain Ceramic Thermal Barrier Coating On Polished Substrates)by D. Ruckle and N. Ulion filed on even date herewith.

The superalloy art has long sought to combine the properties of ceramicswith the properties of metals. Thus for example, many attempts have beenmade to provide protective ceramic coatings to metal articles which findapplication at elevated temperatures so as to combine the thermalproperties of ceramics with the ductility of metals.

The primary problem which has not been successfully solved heretofore isthat the substantial difference in the coefficient of thermal expansionof metals and ceramics invariably leads to failure of ceramic coatingsunder conditions of severe thermal cycling.

One approach which has been employed in an effort to overcome thisproblem is that of grading the coating from essentially all metal at themetal surface to all ceramic at the outer surface of the coating. Inthis way it is believed that the coefficient of thermal expansion willchange gradually through the coating thickness and the stress resultingfrom thermal cycling will not be sufficient to cause damage to thecoating. Such an approach is described in U.S. Pat. No. 3,091,548 toDillon. The problem with the graded approach is that the discrete metalparticles in the graded coating oxidize and increase in volume producingunacceptable stresses in the coating.

In the general area of metal-ceramic combinations, it is known to usesegmented ceramic pieces such as tiles which are bonded to metalstructures for their protection. In this approach, which is generallyapplied to large articles, the segments are not bonded to each other,and the gaps between the tiles permit accommodation of the thermalexpansion of the metal. Such an approach (the application of individualsegments) would not be practical in the case of gas turbine enginecomponents in which extreme operating conditions will be encountered andwhich a multiplicity of small complex parts must be coated at areasonable cost. Additionally, in the use of such a segmented ceramicapproach, there still remains the problem of obtaining a goodmetal-ceramic bond.

In a different art area, it is known to apply coatings of ceramics andmetals by vapor deposition. The general subject of vapor deposition isdescribed in an article by R. F. Bunshah "Journal of Vacuum Science ofTechnology," Vol. 11, No. 4, July/August 1974. The application ofceramics by vapor deposition is employed chiefly in the semiconductorand optics industries where extremely thin coatings are used.

In vapor deposition, the article to be coated is held over a molten poolof material of appropriate composition which evaporates, and the vaporcondenses on and coats the article. This process is used in a variety ofapplications including the application of metallic coatings to gasturbine engine parts. The application to gas turbine engine parts isdescribed in the "Journal of Vacuum Science of Technology," Vol. 11, No.4, July/August 1974, pgs. 641 through 646 in an article by Boone et al.

This article also describes the types of defects which can occur invapor deposited coatings. The most significant defect described istermed a "columnar defect" in which the coating forms as columnar grainswhich are poorly bonded to each other. Such a structure is described asbeing detrimental because the exposed columnar surface greatly increasedthe surface exposed to the environment and because the gaps between thecolumns may adversely affect mechanical properties. The articleindicates that practical uses of vapor deposited coatings requires thatthe columnar type of structure be minimized.

A paper entitled "High Rate Sputtered Deposition of Protective Coatingson Marine Gas Turbine Hot Section Superalloys," authored by J. Fairbankset al was presented in July 1974 at a conference on "Gas TurbineMaterials in the Marine Environment" and was subsequently presented as areport by the Metals Information Center of the Department of Defense(MCIC 75-27). The paper indicates that columnar growth defects wereobserved in sputtered ceramic coatings. The paper hypothesizes that acoating with a columnar structure might permit stress relaxation of thecoating and thereby enhance coating life.

Subsequent development of this concept by one of the authors is detailedin NASA Report NASA-CR-159412 issued July 19, 1978. This reportdescribes the sputter deposition of zirconia based columnar coatings oncopper substrates. The investigation was not successful in producing acoating which could withstand cycling between -196° F. and 400° F. Theinvestigators also performed experiments in which a titanium interlayerwas deposited prior to the ceramic deposition. However, the resultantcoatings spalled under conditions of moderate thermal cycling. In theconclusion of the report, the investigator indicated that the coatingperformance was substantially less than that observed in previous workusing graded coatings. The same investigator also performed work for theNaval Sea Systems Command on graded metal-ceramic coatings applied bysputtering in which columnar coatings were produced. These coatings wereunsuccessful in resisting spalling under conditions of severe thermalcycling. The report is entitled "Develop Sputter Deposited Graded MetalZrO₂ Coating Technology for Application to Turbine Hot SectionComponents," Contract No. N00024-75-C-4333, Oct. 11, 1976.

Ceramic coatings have also been applied by a plasma spray process. Themost successful plasma spray coatings to date have been applied toarticles which have been previously coated with a metallic bond coat.Among the bond coats investigated have been the MCrAlY class ofmaterials. In this situation, the bond coat appears to function byacting as a soft, rough layer in which the plasma spray particles areembedded forming a mechanical bond. This is described in U.S. Pat. No.4,055,705 and pending application U.S. Ser. No. 811,807 which pendingapplication has been the prevailing application in Interference No.100,011.

DISCLOSURE OF INVENTION

The present invention includes a composite coating system which protectsmetallic articles from environmental damage especially under conditionsof high temperature. The novel application method also forms a part ofthe present invention.

The article to be protected is supplied with a uniform adherent MCrAlYlayer. On this MCrAlY layer, there is applied a ceramic coating having aparticular novel columnar microstructure.

The ceramic coating is comprised of many individual columnar segmentswhich are firmly bonded to the article to be protected, but not to eachother. By providing gaps between the columnar segments, the metallicsubstrate may expand without causing damaging stresses in the ceramic.

The ceramic coating is applied by a vapor deposition process. Acontinuous alumina layer is present between the MCrAlY component and thecolumnar ceramic coating. This alumina layer plays a crucial role inbonding the ceramic coating to the MCrAlY layer.

BRIEF DESCRIPTION OF DRAWINGS

The details of the invention will be described along with theaccompanying drawings.

FIG. 1 is a cross sectional line drawing showing the invention coatingand

FIGS. 2 and 3 are a photomicrographs which show experimental coatings.

BEST MODE FOR CARRYING OUT THE INVENTION

The thermal barrier coating system of the present invention is acomposite coating which includes three interrelated elements whichperform different functions. The performance of the coating system issuperior to that of any other known high temperature coating whenevaluated in gas turbine engine environments. The invention coatingsystem provides oxidation and corrosion protection equal to that of bestcurrent coatings in combination with significant thermal barrier orinsulating capabilities.

The major use of the invention coating is in the protection ofsuperalloy articles. Superalloys are nickel, cobalt, and iron basealloys which have exceptional properties at elevated temperatures.Typical compositions are listed in Table 1.

                                      TABLE 1                                     __________________________________________________________________________           Cr                                                                              Co Al                                                                              Ti                                                                              Mo W   Ta                                                                              Cb                                                                              V C  Fe                                                                              Ni                                          __________________________________________________________________________    IN 100 10                                                                              15 5.5                                                                             4.7                                                                             3.0                                                                              --  --                                                                              --                                                                              1.0                                                                             .18                                                                              --                                                                              Bal                                         MAR M200                                                                              9                                                                              10 5.0                                                                             1.0                                                                             -- 12.5                                                                              --                                                                              1.0                                                                             --                                                                              .15                                                                              --                                                                              Bal                                         MAR M509                                                                             24                                                                              Bal                                                                              --                                                                               .2                                                                             -- 7   7.5                                                                             --                                                                              --                                                                              .6 1.0                                                                             10                                          WI 52  21                                                                              Bal                                                                              --                                                                              --                                                                              -- 11  --                                                                              2 --                                                                              .45                                                                              2 --                                          __________________________________________________________________________

The invention thermal barrier coating has a major use in gas turbineengines and was developed with this application in mind. However, thereare many other potential applications for which this coating or somevariation thereof would be well-suited.

The coating consists of a metallic layer of MCrAlY alloy, a continuousadherent alumina layer (formed in situ) on the metallic layer and adiscontinuous pure ceramic layer of a particular columnar morphology onthe alumina layer.

The metallic layer is comprised of a MCrAlY alloy which has a broadcomposition of 10 to 30% chromium, 5 to 15% aluminum, 0.01 to 1% yttrium(or hafnium lanthanum, cerium and scandium) with M, being the balance,being selected from the group consisting of iron, cobalt, nickel andmixtures thereof. Minor amounts of other elements may also be present.Such alloys are known in the prior art for use alone as a protectivecoating and are described in various U.S. Pat. Nos. including 3,542,530;3,676,085; 3,754,903 and 3,928,026 which are incorporated herein byreference.

This invention also contemplates the use of various interlayers betweenthe superalloy substrate and the MCrAlY layer. In particular, it isknown from U.S. Pat. No. 4,005,989 that the use of an aluminide layer(produced by aluminizing) between a substrate and a MCrAlY layer canprovide improved coating durability. Other materials such as platinumhave also been proposed for interlayer use. Of course, such interlayerswill be used only where necessary and only where they do not adverselyaffect the bond between the substrate and the MCrAlY.

It is preferred that this MCrAlY layer be applied by vapor desposition.Such a deposition process in combination with peening and heat treatingprovides a dense adherent layer of relatively uniform thickness which isbasically free from defects. A thickness of 1-10 mils is suitable.

Other deposition processes may be employed for producing the MCrAY layerincluding sputtering and plasma spraying, possibly with associated postcoating treatments, so long as they produce a uniform thickness highintegrity coating of the desired composition.

The alumina layer on the MCrAlY layer is produced by oxidation of theMCrAlY layer. This oxide layer is relatively thin (0.01-0.1 mil),uniform and adherent. Adherence of the oxide layer is greatly improvedin MCrAlY alloys compared to that of similar alloys which do not containyttrium or similar active elements. This improved adherence results fromthe formation of yttrium oxides which extend into the MCrAlY and arebonded to the alumina surface layer thus anchoring the surface layer andminimizing spalling.

The adherence of the alumina layer is essential to the adherence of thecolumnar ceramic layer and the presence of yttrium or equivalent oxygenactive elements such as lanthanum, cerium, hafnium, and scandium ormixtures or oxide particles thereof, in the metallic coating, isimportant to the proper functioning of the invention coating system.

The final component of the thermal barrier coating is a unique columnargrained ceramic surface coating which is tightly bonded to the aluminalayer. The columnar grains are oriented substantially perpendicular tothe surface of the substrate with free surfaces between the individualcolumns extending down to the aluminum oxide layer.

The columnar ceramic surface layer is a pure ceramic as distinguishedfrom some prior art which has suggested the use of a graded layerincorporating substantial amounts of metal in the coating.

The columnar nature of the surface layer circumvents the difference inthe coefficients of thermal expansion between the substrate and thecoating which is believed responsible for failure in prior art ceramicthermal barrier coatings. Upon heating, the substrate expands at agreater rate than the ceramic surface coating and the columnarboundaries between the individual ceramic columns open to accommodatemismatch strains. This limits the stress at the interface between thesubstrate and the columnar ceramic to a level below that which willproduce a fracture of a columnar surface layer. The columns havedimensions on the order of 0.1 mil in cross section.

The columnar surface layer may be any of many ceramic compositions. Mostof the experimental work to date has been performed with a ceramiccomposed of zirconium oxide stabilized by the addition of either 20 or35% yttria to ensure a cubic structure at all temperatures of interest.

It is difficult to specify exactly the characteristics required in theceramic material used as the columnar coating. It appears that thereshould be some degree of solid solubility between the columnar ceramicmaterial and alumina. This is believed to be the major criteria whichmost affects the adherence of the columnar ceramic coating to thealumina layer. Other characteristics are also necessary. The columnarceramic material should not form low melting compounds (e.g. eutectics)when in contact with alumina at elevated temperatures. The melting point(and sublimation point) of the columnar ceramic material should besubstantially greater than the service temperature.

Finally, the columnar ceramic material should be stable in the intendedenvironment; i.e., the material should not oxidize or otherwise reactwith the environment to any significant extent (some ceramics such asSi₃ N₄ will oxidize at elevated temperatures but the oxidation is selflimiting since the oxide produced (SiO₂) protects against furtheroxidation). The following ceramics are believed to have utility as thecolumnar coating material of the present invention: zirconia (preferablystabilized with a material such as yttria), alumina, ceria, mullite,zircon, silica, silicon nitride, hafnia, and certain zirconates, boridesand nitrides.

In summary, therefore, the columnar ceramic material should have somedegree of solid solubility in alumina and should be stable in theintended use environment. I believe that the skilled artisan will haveno difficulty in selecting an appropriate ceramic based on the previousguidelines.

The function of the MCrAlY layer is to adhere strongly to the substrateand to produce a strong adherent continuous oxide surface layer. Thealumina surface layer so-produced protects the underlying MCrAlY layerand substrate against oxidation and hot corrosion and provides a firmfoundation for the columnar grain ceramic surface layer.

The columnar grain ceramic surface layer reduces the temperature of theunderlying substrate and coating layers. Because of the nature of manyceramics and the existence of the open boundaries between the columns,the ceramic surface layer is relatively transparent to oxygen and doesnot play a major role in reducing the oxidation of the underlying layersexcept to the extent that the reduction in the temperature of theunderlying layers reduces the rate of oxidation. Preliminary indicationsare that a 5 mil thick ZrO₂ base coating can reduce substratetemperatures by from 50° to 200° F. under conditions typical of thosefound in current gas turbine engines with cooled blades.

The ceramic surface layer is also believed to play a role in reducinghot corrosion by acting as a barrier between the underlying MCrAlY layerand the various liquid and solid combustion products which can cause hotcorrosion. The ceramic layer is also believed to be beneficial inprotecting against hot corrosion by increasing the rate of evaporationof surface deposits in certain circumstances as a result of the highsurface temperature of the ceramic which results from its thermalinsulation capabilities.

FIG. 1 shows a cross sectional line drawing of a coating according tothe present invention. The substrate material 1 is coated with an MCrAlYlayer 2. On this layer 2, there is formed an adherent alumina layer 3.Finally, a columnar ceramic layer 4 adheres to the alumina layer 3. FIG.2 shows a representative photomicrograph. The numbers on FIG. 2correspond to the numbers on FIG. 1.

Having described the structure of the coated article, we will nowdescribe a preferred method of producing this coating on gas turbinecomponents such as blades and vanes.

The initial step is the preparation of the surface to be coated. Thesurface must be clean of all dirt, grease, oxides and the like.

The cleaning method I have used is vapor honing in which an aqueousabrasive slurry is propelled against the surface to be cleaned withsufficient force to remove all extraneous material from the surface.Following this step, the surface is preferably vapor degreased. Whilethis is a satisfactory cleaning process, numerous alternative processesare possible.

Next, the MCrAlY layer is applied. It is preferred that this MCrAlYlayer be applied by vapor deposition. The deposition process isperformed by holding the surface to be coated over a pool of moltenMCrAlY material in a vacuum chamber. The heat source used to keep theMCrAlY molten is usually an electron beam.

The surface to be coated is preferably maintained at a temperature ofabout 1600°-1800° F. during the MCrAlY deposition process.

It is preferred that the MCrAlY layer have a thickness of about 1 toabout 10 mils. MCrAlY thicknesses below about 1 mil do not provideadequate protection to the surface and thicknesses in excess of about 10mils are prone to rippling during repeated thermal cycling.

In conventional MCrAlY practice, the coatings are dry glass bead peenedto densify any voids and to improve the coating structure. Such peeningis preferred, but has not been found essential.

The coating is then preferably heat treated at 1975° F. in hydrogen,however neither the time or temperature is particularly critical. I haveused a 4-hour treatment to improve the adherence of the coating to thesubstrate.

In the particular preferred processing sequence, this hydrogen heattreatment also serves to develop the desired alumina layer. Thisoxidation occurs as a result of oxygen impurities in the hydrogen. Ihave also employed a separate oxidation step in air in the temperaturerange of about 500°-2000° F. and the results appear to be similar. Thisthermal oxidation process represents the preferred method of aluminadevelopment.

It also appears possible to develop the alumina layer after the depositof the columnar grained ceramic layer. This is especially likely in thecase of zirconia based ceramics which are quite transparent to oxygen.However formation of the alumina layer prior to the columnar grainedceramic layer deposition is preferred.

Following the application of the MCrAlY layer and the development of theoxide layer, the columnar grained ceramic surface layer is applied by avapor deposition process.

The ceramic to be deposited is melted and maintained as a molten pool orevaporation source. We have used 10-20 mesh ceramic powder as a startingmaterial although other starting forms are also satisfactory. Thesubstrate to be coated is positioned over the evaporation source and ismanipulated to produce a uniform coating thickness and to enhance theproduction of a columnar structure. The ceramic coating thickness mayrange from about 1 to about 50 mils.

During the ceramic coating cycle, it has been found desirable tomaintain the substrate at a relatively low temperature, e.g.,1000°-1500° F. to provide a relatively coarse columnar structure and arelatively stoichiometric coating composition.

The coating of the invention is novel in the sense that the prior arthas, in general, gone to some lengths to avoid the production of acolumnar structure with intercolumnar gaps which have been regarded ascoating defects. This invention utilizes what has hereto fore beenregarded as a coating defect to provide improved coating performance.

The present invention will be more readily understood by reference tothe following illustrative examples.

EXAMPLE 1

The component which was coated was a first stage turbine blade from aJT9D gas turbine engine, a commercial aircraft engine of about 50,000pounds thrust.

The blade was composed of alloy MAR M200 plus hafnium which has anominal composition of 9% chromium, 10% cobalt, 12.5% tungsten, 1%columbium, 2% titanium, 5% aluminum, 1.5% hafnium, 0.015% boron, 0.05%zirconium and 0.15% carbon.

In preparation for the application of MCrAlY coating, the cast blade wascleaned by a vapor honing using an aqueous slurry of -200 mesh silicapropelled at a pressure of about 60 psi against the surface of theblade.

After vapor honing, the blade was vapor degreased and a 5 mil coating ofNiCoCrAlY having a nominal composition of 18% chromium, 23% cobalt,12.5% aluminum, 0.3% yttrium, balance nickel was deposited by electronbeam vapor deposition in a vacuum chamber.

After application of the NiCoCrAlY coating, the blade was peened usingfine glass beads propelled by air pressure. This process densifies thecoating. The surface roughness after peening was 35-40 rms. After thepeening, the coated article was heat treated at 1975° F. for 4 hours inhydrogen and cooled to room temperature. During this heat treatment analumina layer formed on the MCrAlY coating.

Following the processing of the MCrAlY coating, the columnar ceramic wasapplied by vapor deposition; during coating the substrate was held atabout 1,000° F. The ceramic applied was cubic ZrO₂ stabilized by 20weight percent Y₂ O₃. The starting ceramic was a 10-20 mesh powder.

The ceramic was melted by an electron beam and was maintained in a poolabout 1 inch diameter. The blade was held about 5 inches over the pooland was rotated over the pool at a rate of about 3 rpm for a period ofabout 4 hours.

The resultant ceramic coating was about 5 mils in thickness and each ofthe columns had an average cross-sectional area of about 0.01 sq. mil.An optical micrograph of this ceramic coating structure is shown as FIG.3.

These coated blades were then heat treated at 1600° F. for 32 hours. Thepurpose of this heat treatment was to assure that the ZrO₂ contained thestoichiometric amount of oxygen. The as deposited ceramic had a darkcoloration, but after the heat treatment the ceramic layer had the lightcoloration which is typical of the stoichiometric ceramic.

The coated blades were installed in a test engine for an evaluation. Theblades have accumulated approximately 100 hours of engine operating timeand visual observation shows that the coating is in good condition.

EXAMPLE 2

Samples were coated and evaluated in a burner rig cycle thermal test. Inthis test, samples are mounted on a rotating platform and a flame formedby the combustion of jet fuel is impinged upon the samples. Aninstrument and control system is used to maintain the samples at acontrolled temperature.

In the cycle used in this example, the samples were heated to and heldat 1850° F. for 4 minutes and then the flame was removed and a jet ofcooling air was applied to cool the samples to below 400° F. in 2minutes. This cycle was repeated until some sign of coating failure wasobserved.

One set of samples was prepared according to the preceding exampleexcept that no MCrAlY layer was applied, instead the vapor depositedcolumnar ceramic layer was applied directly to the clean superalloysubstrate. The superalloy used was the same MAR M200 previouslydescribed.

The second set of samples was prepared using a plasma spray process todeposit a 0.005 inch thick layer of ZrO₂ on a 0.005 inch NiCoCrAlY layerwhich had been applied by vapor deposition. This process is consistentwith the teachings of allowed U.S. application Ser. No. 811,807.

The third set of samples was prepared according to the presentinvention. A 0.005 inch thick layer of NiCoCrAlY was applied by vapordeposition and a 0.005 inch thick layer of ZrO₂ was subsequently appliedby vapor deposition. The processing sequence was identical to thatdescribed in the previous example.

The results of the burner rig tests using the previously described cyclewere as follows. The first set of samples, those samples without theMCrAlY interlayer, failed in less than 200 cycles or 20 hours oftesting.

The second set of samples, those in which the ceramic applied by plasmaspraying, failed after 1210 cycles or 121 hours of burner rig testing.

The third set of samples, those made according to the present invention,have withstood more than 29,000 cycles or 2900 hours of testing withoutfailure and the test is continuing.

The first two sets of samples failed by spallation of the ceramic fromthe sample. Comparison of the first sample set and third sample setclearly indicates the necessity of the MCrAlY intermediate layer ifadherence of the ceramic layer is to be obtained. It appears that theprovision of a MCrAlY interlayer improves the performance and adherenceof the ceramic coating by a factor in excess of 100.

A comparison of the second and third set of samples demonstrate thesignificance of the particular ceramic structure and means ofapplication of the ceramic layer. It can be seen that applying acolumnar ceramic layer by vapor deposition improves the coating life bya factor of at least 20 over the life of a ceramic layer of the samecomposition applied by plasma spray techniques.

I claim:
 1. A superalloy article having an adherent durable ceramic thermal barrier coating including:a. a superalloy substrate, b. an adherent dense coating consisting essentially of MCrAlY on the substrate where M is selected from the group consisting of iron, nickel, cobalt and mixtures of nickel and cobalt, c. an alumina layer on the MCrAlY coating surface, d. an adherent columnar layer, consisting essentially of ceramic, on the alumina layer.
 2. A coated article as in claim 1 in which the thickness of the MCrAlY is from about 1 to about 10 mils.
 3. A coated article as in claim 1 in which the columnar ceramic coating has a thickness of from about 1 to about 50 mils.
 4. A superalloy article having an adherent durable ceramic thermal barrier coating including:a. a superalloy substrate, b. an adherent dense coating consisting essentially of MCrAlY substrate, where M is selected from the group consisting of iron, nickel, cobalt and mixtures of nickel and cobalt, c. an alumina layer on the MCrAlY coating surface, having a thickness of from about 0.01 to about 0.1 mils, d. an adherent columnar layer, consisting essentially of ceramic, on the alumina layer.
 5. An article as in claim 1 or 4 in which the columnar ceramic consists essentially of stabilized zirconia. 